This application relates to aircraft control systems and more particularly to aircraft attitude estimation with reduced or compromised sensor data.
An aircraft is a vessel that is free to move in three dimensional space. FIG. 1 depicts a typical coordinate system useful for describing aircraft motion in three dimensions. In the body fixed coordinate system of FIG. 1, the aircraft has a longitudinal axis xb which extends along the length of the airplane. Rotation about the xb axis, L ,is called roll. The coordinate system of FIG. 1 further includes a lateral axis yb extending parallel to the aircraft wing. Rotation about the yb axis, M, is called pitch. The zb axis extends perpendicular to the remaining axes as shown. Rotation about the zb axis, N, is called yaw.
Equations of motion can be derived to describe the aircraft movement using the axes shown in FIG. 1. Unfortunately, the orientation and position of the aircraft in space cannot be truly understood with the coordinate system of FIG. 1 since the coordinate system is moving with and is always centered on the body of the aircraft. For this reason, it is common to transform the parameters of FIG. 1 to describe the angular displacement of the airc raft in space. These angular displacements, or Euler angles, are as shown in FIG. 2.
In good weather, under visual flight conditions, pilots of conventional aircraft control the aircraft motions and the resulting angular displacements in three dimensional space by visual reference to the natural horizon. The natural horizon serves as a visual clue from which the pilot can determine if the airplane is climbing, descending or turning. In low visibility conditions, such as, for example: nighttime, haze, or flight in clouds; the natural horizon can become obscured and the pilot is unable to control the aircraft by reference to the natural horizon. Conventional aircraft are therefore equipped with several instruments to assist the pilot in visualizing the aircraft""s movement in three dimensional space. These instruments also provide the pilot with supporting data from which to confirm control of the aircraft even when the natural horizon is visible.
FIGS. 3A-3G show a conventional aircraft panel for a contemporary airplane having such standard instrumentation. The control panel of FIGS. 3A-3G include: an altimeter 2 that provides the pilot with information on aircraft altitude; an airspeed indicator 4, that provides information on the aircraft speed through the air; and a vertical speed indicator 6, that provides data on the rate of climb and descent. Instruments 2, 4 and 6 comprise the pitot-static, or pneumatic, instruments since they operate by sensing air pressures exterior to the aircraft. In certain larger aircraft, the pitot static instruments sensors are combined into a single box called an air data computer. The air data computer then outputs the altimetry and airspeed data to a cockpit display and/or to other avionics equipment requiring such data.
Also included in the standard control panel of FIGS. 3A-3G are the gyroscopic instruments. The gyroscopic instruments provide the pilot with a pictorial view of the airplane""s rate of turn, attitude and heading. These instruments include a turn coordinator 8, an attitude indicator 10, and a heading indicator 12. A wet magnetic compass 13, may also be used to provide heading information. Wet compass 13 does not contain a gyro.
FIGS. 4A-4B illustrate aircraft turn coordinator 8 in greater detail. Turn coordinator 8 senses yaw, r ,and roll, p, movement about the aircraft Zb and Xb axes. When the miniature airplane 14 is level as shown in FIG. 4A, the aircraft is neither turning nor rolling. When the aircraft banks, miniature airplane 14 also banks. In the drawing of FIG. 4B, miniature airplane 14 indicates a turn to the right.
FIGS. 5A-5D illustrate operation of aircraft attitude indicator 10 also known as an artificial horizon. Attitude indicator 10 senses pitching, "THgr", and rolling, xc3x8, movements at out the airplane""s lateral and longitudinal axes. Attitude indicator 10 is the only flight instrument that provides both pitch and bank information to the pilot. Attitude indicator 10 presents a view of the aircraft, as represented by miniature airplane 20, as the aircraft would appear to someone standing behind it. The pitch attitude of the aircraft is shown by noting the position of the nose 22 of miniature airplane 20 relative to the artificial horizon 24. Bank information is shown both by noting the position of miniature airplane 20 relative to the deflected artificial horizon 24 and by the alignment of bank angle pointer 28 with the graduated bank angle indexes located on the perimeter of the device. FIG. 5A shows the aircraft in level flight and no turn. FIG. 5B shows the aircraft in a level turn to the left. FIG. 5C shows a level climb and FIG. 5D shows a descending left turn.
Heading indicator 12, also known as a directional gyro, serves as a means to indicate the aircraft magnetic heading without the limitations of using wet compass 13. Wet compass 13 is prone to various turning and acceleration errors due to the interaction of the magnetic compass with the earth""s magnetic field. Heading indicator 12 is not subject to these errors and thus provides the pilot with a more stable indication of aircraft heading throughout the flight.
Each of turn coordinator 8, attitude indicator 10, and heading indicator 12 includes a gyroscope needed for proper operation of these instruments. Typically, the gyroscopes in attitude indicator 10 and heading indicator 12 are powered by a vacuum pump. Turn coordinator 8 is normally powered using an electric motor. The gyroscopes contained within each of these instruments also have operating limitations. For example, if the aircraft enters an extreme or unusual flight attitude, the gyroscope can tumble rendering the associated instrument inoperative.
Similar to the air data computer, the gyroscopic instruments are occasionally on larger aircraft combined into a single integrated sensor package called an attitude heading reference system, or AHRS. The AHRS system outputs the attitude data to a cockpit display and to other avionics equipment requiring such data.
If either the gyroscope within the individual AHRS sensor instrument or the power source for the AHRS gyro instrument fails, the pilot no longer can rely on the affected instrument(s) for navigation and control of the aircraft. In such situations, pilots are taught to fly xe2x80x9cpartial panelxe2x80x9d in which the pilot mentally integrates information from the remaining instruments to supply the information normally supplied by the missing instrument. This mental integration task is demanding, especially for an inexperienced or out of practice pilot. Numerous accidents have resulted when the pilot was forced to fly partial panel.
In airplanes with autopilots, the autopilot uses the attitude information supplied by these gyroscopic instruments to fly the aircraft. In certain aircraft, without redundant instrumentation, these gyroscopes supply the only sensor inputs used to operate the autopilot. Thus, when an instrumentation failure occurs, the autopilot becomes inoperable as well.
The present invention recognizes that in the event of instrument failure, it would be desirable to supply the pilot with estimates of the omitted information such that the pilot is not required to perform the mental integration task. The present invention thus contributes to improved aircraft safety by reducing pilot workload and fatigue in the event of instrument failure.
The present invention further enables continued autopilot operation in the event of a single gyro or entire gyro system failure. The architecture of the present invention thus enhances the robustness of the autopilot without the added weight, cost and maintenance expense of a redundant sensor set.
The present invention also provides a back up system to the AHRS system when the aircraft is so equipped. According to one aspect of the present invention, the invention provides a method, apparatus and computer program product useful for estimating airplane pitch, roll and heading angles using a reduced set of sensors. The present invention exploits characteristics of aircraft kinematics, together with sensor characteristics, filtering and estimation techniques as well as simplifying assumptions based on flight regime to obtain angular estimates desired by the pilot for continued controlled operation of the aircraft.
According to another aspect of the invention, the invention includes a signal processing device further including logic, either software and/or hardware, for estimating aircraft roll angles, pitch angles and heading angles. Each angle estimator is coupled to a suite sensors from which the desired angle may be estimated in the absence of sensor data directly measuring that angle. The sensors may be integrated with the present invention or located separately onboard the aircraft.
According to another aspect of the present invention, the invention additionally includes an interface for displaying the angular estimates obtained to the pilot. In a preferred embodiment of the invention, this interface may comprise an electronic cockpit display, a driver for displaying ansular data on an existing cockpit display and/or a converter for driving a mechanical ansular display instrument existing in the cockpit.
According to still another aspect of the present invention, the roll angle, pitch angle and heading angle estimators may be separate units and need not be bundled as an integrated package.
Further details and operation of the invention are described below.